Age differences are ubiquitous and large in studies of the speed with which individuals execute cognitive processes. These age differences may reflect acquisition of task-specific skills or they may reflect change in a central mechanism that limits performance on all speeded tasks. Three types of experiments and proposed to provide evidence to distinguish these explanations. On approach is to examine developmental functions for speeds of different processes. If a central mechanism limits processing speed, speeds of distinct processes should develop at the same rate; if task-specific skills are responsible for age differences in processing speed, there is no necessary relation between rates of development for different processes. These productions will be evaluated by comparing developmental functions derived from performance on four tasks: mental arithmetic, mental addition, figural matrices, and geometric analogies. The subjects will be 8- to 22-year-olds. A second approach is to examine processing speed in domains where children and adults have equal knowledge. Here, age differences should be eliminated according to the specific-skills explanation, but only reduced according to the central mechanism hypothesis. These predictions will be tested by comparing the performance of child and adult baseball experts as well as child and adult novices on visual search, memory search, and mental rotation tasks. A third approach is to have subjects perform two tasks simultaneously. According to the specific skills view, if children and adults are equal in their performance of the tasks individually, they should have equal concurrent performance on the tasks. In contrast, according to the central mechanism hypothesis, children's concurrent performance should be less than adults', reflecting the greater constaints imposed on children by the central mechanism. These predictions will be evaluated by testing children and adults on mental rotation, visual search, or memory search tasks. Each task will be performed with and without a concurrent task.